Garden Railway Forum

A question that gets raised regularly on this Forum is why the different thread sizes. I do not have an engineering background or training so thought I'd sound out the expert's advice at the Peterborough Show.

1/4" x 26 BSF was (and still is) the standard thread size used by Mamod for their boiler inserts. It is also the same thread size used by MSS on their locomotive boilers. The MSS & original Mamod SL boilers were certified under the Toy Steam regulations to a max of 1 Bar (approx 14.5 PSI). Mamod produced a silver soldered replacement boiler for the SL range which apparently was certified to 30 PSI, again with 1/4" x 26 BSF inserts.

I have a replacement Mike Chaney boiler where the water top up is 1/4" x 26 BSF, but oddly the safety valve is 1/4" x 40 TPI (I'm unaware if he ever produced a replacement safety valve in 1/4" x 26 BSF). This boiler is stamped at 50 PSI.

I have a Samson Locomotive Works locomotive (a much improved Mamod derivative) where the boiler is fitted with 3 x 1/4" x 26 BSF inserts. No boiler pressure details but pressure gauge goes up to 50 PSI.

The IP Jane/IP Eng replacement boiler has 1/4" x 40 ME inserts and operated to 40 PSI (initially 30 PSI). This was replaced by the Dream Steam boiler which now operates up to 45 PSI.

The PPS Janet/Mamod replacement boiler also had 1/4" x 40 TPI inserts. Initially operated to 40 PSI that pressure was increased to 65 PSI. This boiler is now sold by RWM Steam.

So to the question. A novice to Garden Railways enters with an initial limiting budget and buys an MSS or equivalent 2nd/3rd hand Mamod SL locomotive. To increase performance a water top up valve and higher safety valve are purchased requiring them to be 1/4" x 26 BSF. Still being limited performance wise a silver soldered boiler is now purchased, part of it's expensive cost being the requirement to have a 1/4" x 40 TPI water top up valve & safety valve fitted, leaving the 1/4" x 26 BSF fittings to be consigned to the spare parts box or sold 2nd hand to try and offset costs.

Question - Is there any technical reason why a silver soldered Mamod SL/MSS boiler has to be supplied with 1/4" x 40 TPI fittings?

Answer - Roy Wood Models - No, but that was how the boilers were supplied when I took them over and I've continued to supply them that way. 1/4" x 40 TPI is the most common thread used. My response - So it is interesting that Cheddar Models used a non common thread of 1/4" x 32 TPI on their products (operated up to 60 PSI).

Answer - Dream Steam - Initially because that is the way they were previously supplied. However when I then said that it was actually how Ivan Prior sold them, and of course his products started off the Dream Steam ones, I was then told a yarn that Ivan would have known what he was doing and that there is a lot more meat to grip with a 1/4" x 40 TPI fitting compared to 1/4" x 26 BSF fitting. So I then said so why did Cheddar Models use 1/4" x 32 TPI fittings at 60 PSI, to which Paul could not add any further useful information.

Unfortunately Ivan Prior was working flat out on his stall so no chance of a historical question there.

So if you want to know the real answer why your silver soldered boiler and fittings are so expensive then you will need to seek out Mike Chaney and Ivan Prior to ask why they used 1/4" x 40 TPI when 1/4" x 26 BSF quite clearly works OK at 40 PSI with no reported problems, and 1/4" x 32 TPI works quite happily at 60 PSI.

Chris Cairns.

Just a headsup - Alan mentioned to me once in a telephone conversation that the advertised 65psi of his Janet boiler was a typing error and should have been 60psi. I operate mine at 50-60psi but if it hits 80, I instantly cut the fire and open the reg to relive pressure.

As for why all the threads differ? It would seem that Ivor Prior holds the reason, as his IP Jane is the grand-daddy of the improved engines.

One going point about 26tpi and 40tpi is as I was informed: at least you can't balls up and put a weaker 26tpi SV onto a 60psi loco.

Or worse, put a 40-50-60psi set SV onto a brass-boilered engine.

I'm curious though as to this: if IPE Sold the rights to the Jane to Dream Steam, how exactly did PPS get in on the loco production act? [not that I question it of course].

Also, Roy has changed the boiler design since he took over. On his Janets there's now only one plug. On my PPS boiler there's two. Sadly he doesn't have any of the old PPS whistles in stock nor will he be making them.

I think the reason is what you do with the original safety valve. As standard you have to take the safety valve out to fill the boiler so it would make sense to use a coarse thread that is harder to damage. The upgrade boilers have the top up valve as a standard fitting usualy so its very rare a fitting will need to be removed.

dougrail:82839 wrote: I'm curious though as to this: if IPE Sold the rights to the Jane to Dream Steam, how exactly did PPS get in on the loco production act? [not that I question it of course].

I'm no expert in the Jane history, however my understanding is:

IP Eng produced the Jane which PPS somehow begun to make them under the name of Janet, whilst Dream Steam never really produced a named loco of sorts, it was more focused on the spares and upgrades (which PPS also produced).

As RWM has bought the old PPS rights to produce them and is excelling himself with the forward motion of what he's done recently, would it be safe to assume that just changing the name of the product being made and a few different specifications to the original PPS essetially allowed themselves to produce the Janet?

I'm sure someone will correct me in my theory and that I've thought way too much into it!

However, back OT regarding the threads, as I've mentioned in the past on here and as Doug said, I always assumed that the reason for having the different threads is to avoid fitting a 40psi safety valve into a 26psi boiler.

I'd say what James said re wrong SV being put in the boiler - recalling now it was MSN with James that gave me this train of thought. Afterall, the brass boilers were sold for 12-15psi operation - so what pressure max could they stand? 40psi?

I suspect if you blank off the SV the first thing to go under pressure might be the sight glass

True this, but I was also led to believe that the reverser block spring also ated as a backup SV?

Ah, yes, forgot about that. I know for certain that the reverser can't take 40psi...it leaks like a sieve, hence why I had to modify Brits original one (which is still in use) with an extra spring.

IP Engineering were not the first to introduce a Mamod upgrade boiler, but their boiler design did lead to the development of the current Dream Steam & RWM Steam boilers.

The theory of putting the wrong rated safety valve in the wrong boiler is an interesting one but the 60 PSI boiler was not introduced until 2002 by PPS with the introduction of the PPS Jenny, so that theory is not the original reason.

The IP Jane was designed by Ivan Prior in 1994/1995, was initially sold as the Brandbright Jane (with the nice rivet detailed smokebox), and was phased out in 2003. Dream Steam did not take over the rights to the IP Jane, and have never produced an in-house locomotive, only a custom painted MSS side tank locomotive.

PPS used to sell the IP Jane, and indeed there was a lot of co-operation between PPS & IP Eng regarding the various Mamod upgrades (they both used the same photo of an upgraded SL1 to showcase their products). PPS introduced the PPS Jenny in 2002 which externally looked like an upgraded MSS side tank locomotive, but very few were sold. The PPS Janet was introduced in 2003, and whilst externally it looks like an IP Jane it's design offered more improvements. It is now sold as the RWM Janet.

The standard Mamod SL boiler has been hydraulically tested and developed a leak at the sight glass at 120 PSI, which sealed again when the pressure was reduced to 60 PSI. Thus it has been deemed safe for use at 25 PSI (the pressure setting of upgraded safety valves made by the likes of IP Eng) and possibly up to 40 PSI.

PPS Steam Whistle - Doug I assume you mean the one that PPS fitted as an option to the PPS De Winton for a short period, and sold them separately.



Well the good news is it is not a PPS made whistle, it is a PM Research steam whistle so you can either buy it from a USA trader, with the risk of being stung for import duty and the Royal Mail admin fee, or buy it from Forest Classics.

Chris Cairns.

Didn't reaslise that was the whistle. They are very nicely made and does produce a very clear note. If you are thinking about one, I will say they are quite large and I have yet to get mine to work correctly on steam.

Chris Cairns:82859 wrote:IP Engineering were not the first to introduce a Mamod upgrade boiler, but their boiler design did lead to the development of the current Dream Steam & RWM Steam boilers.

The theory of putting the wrong rated safety valve in the wrong boiler is an interesting one but the 60 PSI boiler was not introduced until 2002 by PPS with the introduction of the PPS Jenny, so that theory is not the original reason.

The IP Jane was designed by Ivan Prior in 1994/1995, was initially sold as the Brandbright Jane (with the nice rivet detailed smokebox), and was phased out in 2003. Dream Steam did not take over the rights to the IP Jane, and have never produced an in-house locomotive, only a custom painted MSS side tank locomotive.

PPS used to sell the IP Jane, and indeed there was a lot of co-operation between PPS & IP Eng regarding the various Mamod upgrades (they both used the same photo of an upgraded SL1 to showcase their products). PPS introduced the PPS Jenny in 2002 which externally looked like an upgraded MSS side tank locomotive, but very few were sold. The PPS Janet was introduced in 2003, and whilst externally it looks like an IP Jane it's design offered more improvements. It is now sold as the RWM Janet.

The standard Mamod SL boiler has been hydraulically tested and developed a leak at the sight glass at 120 PSI, which sealed again when the pressure was reduced to 60 PSI. Thus it has been deemed safe for use at 25 PSI (the pressure setting of upgraded safety valves made by the likes of IP Eng) and possibly up to 40 PSI.

PPS Steam Whistle - Doug I assume you mean the one that PPS fitted as an option to the PPS De Winton for a short period, and sold them separately.



Well the good news is it is not a PPS made whistle, it is a PM Research steam whistle so you can either buy it from a USA trader, with the risk of being stung for import duty and the Royal Mail admin fee, or buy it from Forest Classics.

Chris Cairns.

Well this is interesting...I was under the impression that Dream Steam "had bought the rights" to the I P E Jane. It seems now that the real 'inheritors' of the IPEJ ane were infact PPS, who in turn have passed it on to RWM.

Now regards the MSS/Mamod brass boiler - it, with a plastic sightlgass, took on 100psi without hassle? Question is, how long could it have maintained that for? Without say, the sight glass bursting , wide blast or piercing-needle style?

Still, to see it could handle 40psi...surprising but not a risk I'd take. They were designed for toy steaming, not to be pushed.

PPS Whistle - I was under the impression it was for te horizontal locos too. It explained why PPS sold their copper boilers with two extra plugs - one was for the pressure gauge and syphon and the other supposedly for the connection to this whistle.

Roy has modified his Janet boilers so now that they only have the one plug for the pressure gauge, but as I bought mine in Dec 2010 from PPS, my engine has two plugs. I'm sort of interested as to whether I might be able to find the whistle and the appropriate connecting pipe but then lies the question - where were they supposed to be situated on a Janet/mamod loco?

Chris Cairns:82859 wrote:The standard Mamod SL boiler has been hydraulically tested and developed a leak at the sight glass at 120 PSI, which sealed again when the pressure was reduced to 60 PSI. Thus it has been deemed safe for use at 25 PSI (the pressure setting of upgraded safety valves made by the likes of IP Eng) and possibly up to 40 PSI.
Chris Cairns.

Remember that temperature is a factor too- for saturated steam temperature and pressure have a fairly precise relationship; as one goes up, so does the other. Cold compressed air at 60 lumps per squinge is one thing but hot steam is quite another. Under that kind of abuse I'd expect the sight glass to soften long before it reached its "normal" breaking strain.